The invention relates to a process for the preparation of catalysts with improved catalytic properties, particularly improved initial activity, initial selectivity and/or activity and/or selectivity performance over time.
Numerous methods are known for the deposition of catalytically reactive metals on a carrier in order to manufacture catalysts. For example, U.S. Pat. No. 3,972,829, issued Aug. 3, 1976, discloses a method for distributing catalytically reactive metallic components on carriers using an impregnating solution of catalyst precursor compound and an organic thioacid or a mercaptocarboxylic acid. U.S. Pat. No. 4,005,049, issued Jan. 25, 1977, teaches the preparation of a silver/transition metal catalyst useful in oxidation reactions. International publication WO 96/23585, published Aug. 8, 1996, teaches that boosting the amount of alkali metal promoter in a silver solution results in improved properties.
Literature also warns against certain methods. U.S. Pat. No. 4,908,343, issued Mar. 13, 1990, warns against having a silver solution which has a strong acidity or basicity as the strongly acid or base solution would leach any leachable impurities from the carrier, becoming part of the silver catalyst in amounts which adversely affects the performance of the catalyst in an oxidation reaction.
It has surprisingly been found that the metal deposition and catalytic properties of a catalyst may be greatly improved by lowering the hydrogen ion activity of the impregnation solution.
According to one embodiment of the invention, there is provided a process for depositing one or more catalytically reactive metals on a carrier, said process comprising:
selecting a carrier; and
depositing a catalytically effective amount of one or more catalytically reactive metals on said carrier, said deposition effected by an impregnating solution wherein a hydrogen ion activity of said impregnation solution is lowered.
There is further provided a process for preparing a catalyst suitable for the vapor phase production of epoxides, said process comprising:
selecting a carrier; and
depositing a catalytically effective amount of silver on the carrier, wherein said deposition is effected by an impregnation solution wherein a hydrogen ion activity of said impregnation solution is lowered.
There is still further provided catalysts made by the processes of the embodiments herein described.
It has been found that lowering the hydrogen ion activity of the impregnation solution used to deposit catalytically reactive metals on a carrier provides catalysts which have improved catalytic properties, such as activity, selectivity and the activity and/or selectivity performance over time. The process is believed to work to improve the properties of most catalysts wherein metal is deposited on a carrier by use of an impregnation solution.
Catalysts are commonly made by depositing a catalytically effective amount of one or more catalytically reactive metals on a carrier to make a catalyst precursor. Typically, the carrier is impregnated with metal or compound(s), complex(es) and/or salt(s) sufficient to deposit or impregnate the catalytically reactive material. As used herein, xe2x80x9ccatalytically effective amountxe2x80x9d means an amount of metal that provides a measurable catalytic effect.
The impregnated carrier, or catalyst precursor, is dried in the presence of an atmosphere which also reduces the catalytic metal. Drying methods known in the art include steam drying, drying in an atmosphere with a controlled oxygen concentration, drying in reducing atmospheres, air drying, and staged drying using a suitable ramped or staged temperature curve.
In the process of the invention, improvement in the catalytic properties are seen when the metal deposition is effected by use of an impregnation solution whose hydrogen ion activity has been lowered. xe2x80x9cHydrogen ion activityxe2x80x9d as used herein is the hydrogen ion activity as measured by the potential of a hydrogen ion selective electrode. As used herein, a solution with xe2x80x9cloweredxe2x80x9d hydrogen ion activity refers to a solution whose hydrogen activity has been altered by the addition of a base, such that the hydrogen ion activity of the altered solution is lowered compare to the hydrogen ion activity of the same solution in an unaltered state. The base selected to alter the solution may be chosen from any base or compound with a pKb lower an the original impregnation solution. It is particularly desirable to choose a base which does not alter the formulation of the impregnation solution; i.e., which does not alter the desired metals concentration in the impregnation solution and deposited on the carrier. Organic bases will not alter the impregnation solution metals concentrations, examples of which are tetraalkylammonium hydroxides and 1,8-bis-(dimethylamino)-naphthalen. If changing the metals concentration of the impregnation solution is not a concern, metal hydroxides may be used.
When the impregnation solution is at least partially aqueous, an indication of the change in the hydrogen activity may be measured with a pH meter, with the understanding that the measurement obtained is not pH by a true, aqueous definition. xe2x80x98xe2x80x9cMeasured pHxe2x80x99xe2x80x9d as used herein shall mean such a non-aqueous system pH measurement using a standard pH probe. Even small changes in the xe2x80x9cmeasured pHxe2x80x9d from the initial impregnation solution to that with added base are effective and improvements in catalytic properties continue as the xe2x80x9cmeasured pHxe2x80x9d change increases with base addition. High base additions do not seem to adversely affect catalyst performance; however, high additions of hydroxides have been seen to cause sludging of the impregnation solution, creating manufacturing difficulties. When the base addition is too low, the hydrogen ion activity will not be affected.
As described, the process is effective in improving at least one of the catalytic properties of catalyst wherein an impregnating solution is used to deposit or impregnate a catalytically reactive metal upon a carrier. xe2x80x9cImprovement in catalytic propertiesxe2x80x9d as used herein means the properties of the catalyst are improved as compared to a catalyst made from the same impregnation solution which has not had the hydrogen ion activity lowered. Catalytic properties include catalyst activity, selectivity, activity and/or selectivity performance over time, operability (resistance to runaway), conversion and work rate.
Further improvement in properties may be achieved by lowering the concentration of ionizable species present on the surface of the carrier prior to the deposition step. Carriers are commonly inorganic materials such as refractory inorganic materials, for example alumina-, silica-, or titania-based compounds, or combinations thereof, such as alumina-silica carriers. Carriers may also be made from carbon-based materials such as, for example, charcoal, activated carbon, or fullerenes. Ionizable species typically present on the inorganic type carriers include sodium, potassium, aluminates, soluble silicate, calcium, magnesium, aluminosilicate, cesium, lithium, and combinations thereof. Lowering the undesirable ionizable species concentration may be accomplished by any means (i) which is effective in rendering the ionizable species ionic and removing that species, or (ii) which renders the ionizable species insoluble, or (iii) which renders the ionizable species immobile; however, use of aggressive medias, such as acids or bases, is discouraged as these media tend to dissolve the carrier, extract too much material from the bulk, and generate acidic or basic sites in the pores. Effective means of lowering concentration include washing the carrier; ion exchange; volatilizing, precipitating, or sequestering the impurities; causing a reaction to make the ionizable species on the surface insoluble; and combinations thereof. Examples of wash and ion exchange solutions include aqueous and/or organic solvent-based solutions which may also contain tetraethylammonium hydroxide, ammonium acetate, lithium carbonate, barium acetate, strontium acetate, crown ether, methanol, ethanol, dimethylformamide, and mixtures thereof. The formed carrier may be treated, or the materials used to form the carrier may be treated before the carrier is manufactured. When the carrier materials are treated before the carrier is formed, still further improvement may be seen by retreating the surface of the formed carrier. Following removal of the ionizable species, the carrier is optionally dried. When the removal process is by washing with an aqueous solution drying is recommended.
By way of example, the process will be described in more detail for a catalyst suitable for the vapor phase production of epoxides, also known as an epoxidation catalyst.
First, a carrier is selected, in the case of an epoxidation the carrier is typically an inorganic material, such as, for example, an alumina-based carrier such as xcex1-alumina. The carrier is typically impregnated with metal compound(s), complex(es) and/or salt(s) dissolved in a suitable solvent sufficient to cause the desired deposition on the carrier. If excess of impregnation solution is used, the impregnated carrier is subsequently separated from the impregnation solution and the deposited metal compound is reduced to its metallic state. In the process of the invention, the hydrogen ion activity of the impregnation solution is lowered prior to beginning the deposition or impregnation process. The typical known impregnation solution for an epoxidation catalyst is quite basic, so in accordance with the present invention a strong base may be used to further lower the hydrogen ion activity. It is particularly desirable to chose a base which does not alter the formulation of the impregnation solution, such as organic bases; however, if changing the metals concentration of the impregnation solution is not a concern, metal bases may be used. Examples of strong bases include alkylammonium-hydroxides, such as tetraethylammonium hydroxide, and metal hydroxides, such as lithium hydroxide and cesium hydroxide. Combinations of bases may also be used. In order to maintain the desired impregnation solution formulation and metal loading, an organic base such as tetraethylammonium hydroxide is preferred. These desired level of base additions typically result in a xe2x80x9cmeasured pHxe2x80x9d change ranging from about 0.5 to about 3, realizing that the xe2x80x9cmeasured pHxe2x80x9d may not be a true pH when the impregnation system is not aqueous. Typically the hydrogen ion activity is lowered such that the xe2x80x9cmeasured pHxe2x80x9d is above 11.2, more typically at least about 11.7, preferably at least about 12.0. Typically the hydrogen ion activity is lowered such that the xe2x80x9cmeasured pHxe2x80x9d is at most about 14.2, more typically at most about 13.7. As defined herein, xe2x80x9cpHxe2x80x9d is deemed to relate to pH measured at 20xc2x0 C.
If an excess of impregnation solution is used, the impregnated carrier is subsequently separated from the solution before the deposited metal compound is reduced. Promoters, components which work effectively to provide an improvement in one or more of the catalytic properties of the catalyst when compared to a catalyst not containing such components, may also be deposited on the carrier either prior to, coincidentally with, or subsequent to the deposition of the catalytically reactive metal.
If the above described ionizable species concentration lowering step is utilized, the concentration of the ionizable species present on the carrier surface is lowered prior to the deposition or impregnation step. Ionizable species present on an xcex1-alumina carrier, for example, typically include sodium, potassium, aluminates, soluble silicates, calcium, magnesium, aluminosilicates, and combinations thereof. It has been found that silicates, and certain other anions, are particularly undesirable ionizable species in an epoxidation catalyst. The solubilization rate of silicates may be measured by inductively coupled plasma (ICP) techniques and the amount of silicon species on a surface may be measured by x-ray photoelectron spectroscopy (XPS); however, since sodium is soluble in the same solutions that silicates are soluble in, the solubilization rate of sodium becomes a simpler check of the ionic species removal. Another measurement technique is to measure the electrical conductivity of the treatment solution.
The concentration of the undesirable ionizable species may lowered by any means which is effective in rendering the ionizable species ionic and removing that species, or rendering the ionizable species insoluble, or rendering the ionizable species immobile. Means effective in lowering the concentration of the undesirable ionizable species on the surface include washing, ion exchange, volatilization, precipitation, sequestration, impurity control and combinations thereof. Cleansing of an alumina-based carrier may be efficiently and cost-effectively accomplished by washing or ion exchange. Any solution capable of reducing the concentration of the undesirable ionizable species present, particularly the anionic ionizable species, and most particularly ionizable silicates, may be used. The carrier is then optionally dried; however, when the removal process is by washing, drying is recommended.
Promoters may also be deposited on the carrier either prior to, coincidentally with, or subsequent to the deposition of the metal(s). As used herein, the term xe2x80x9cpromoterxe2x80x9d refers to a component which works effectively to provide an improvement in one or more of the catalytic properties of the catalyst when compared to a catalyst not containing such component. Promoters are typically compound(s) and/or salt(s) of alkali metal which are optionally deposited on the carrier either prior to, coincidentally with, or subsequent to the deposition of the catalytically reactive metal. Promoters may include, for example, sulfur, phosphorus, boron, fluorine, Group IA through Group VIII metals, rare earth metals, and combinations thereof.
The carrier having the controlled solubilization rate is impregnated with metal ions or compound(s), complex(es) and/or salt(s) dissolved in a suitable solvent sufficient to cause the desired deposition on the carrier. When silver is the deposition material, a typical deposition is from about 1 to about 40 percent by weight, preferably from about 1 to about 30 percent by weight silver, basis the weight of the total catalyst. The impregnated carrier is subsequently separated from the solution and the deposited metal(s) compound is reduced to metallic silver.
One or more promoters may be deposited either prior to, coincidentally with, or subsequent to the deposition of the metal. Promoters for epoxidation catalysts are typically selected from sulfur, phosphorus, boron, fluorine, Group IA through Group VIII metals, rare earth metals, and combinations thereof. The promoter material is typically compound(s) and/or salt(s) of the promoter dissolved in a suitable solvent.
For olefin epoxidation catalysts, Group IA metals are typically selected from potassium, rubidium, cesium, lithium, sodium, and combinations thereof; with potassium and/or cesium and/or rubidium being preferred. Even more preferred is a combination of cesium plus at least one additional Group IA metal, such as cesium plus potassium, cesium plus rubidium, or cesium plus lithium. Group IIA metals are typically selected from magnesium, calcium, strontium, barium, and combinations thereof, Group VIII transition metals are typically selected from cobalt, iron, nickel, ruthenium, rhodium, palladium, and combinations thereof; and rare earth metals are typically selected from lanthanum, cerium, neodymium, samarium, gadolinium, dysprosium, erbium, ytterbium, and mixtures thereof. Non-limiting examples of other promoters include perrhenate, sulfate, molybdate, tungstate, chromate, phosphate, borate sulfate anion, fluoride anion, oxyanions of Group IIIB to VIB, oxyanions of an element selected from Groups III through VIIB, alkali(ne) metal salts with anions of halides, and oxyanions selected from Groups IIIA to VIIA and IIIB through VIIB. The amount of Group IA metel promoter is typically in the range of from about 10 ppm to about 1500 ppm, expressed as the metal, by weight of the total catalyst, and the Group VIIb metal is less than about 360 ppm, expressed as the metal, by weight of the total catalyst.
Other embodiments of the invention provide catalysts made by the processes just described.
The resulting epoxidation catalysts just described are used for the vapor phase production of epoxides. A typical epoxidation process involves loading catalysts into a reactor. The feedstock to be converted, typically a mixture of ethylene, oxygen, carbon dioxide, nitrogen and ethyl chloride, is passed over the catalyst bed at pressure and temperature. The catalyst converts the feedstock to an outlet stream product which contains ethylene oxide. Nitrogen oxides (NOx) may also be added to the feedstock to boost catalyst conversion performance.
Having generally described the invention, a further understanding may be obtained by reference to the following examples, which are provided for purposes of illustration only and are not intended to be limiting unless otherwise specified.